1. Field of the Invention
The present invention relates to a superposed digital signal transmission scheme for transmitting high speed digital signals and low speed digital signals used in a digital communication system.
2. Description of the Background Art
The currently adopted satellite video communication system uses a video signal transmission earth station capable of transmitting both video signals and voice signals and small earth stations capable of transmitting only voice signals, in which the transmission of the video signals is realized as a one-way transmission in an analog FM mode from the video signal transmission earth station, while the transmission of the answer-back voice signals from the small earth station which receives the video signals from the video signal transmission earth station is realized by using a terrestrial network.
However, such a current satellite video communication system has been associated with a problem concerning a limited flexibility available in setting up a new small earth station because of the use of the terrestrial network for the answer-back voice signal transmission.
In order to economize a practical implementation of such a satellite video communication system by improving the flexibility in setting up of a new small earth station, it has been considered desirable to realize a video communication system which fully uses digital satellite transmissions for all the signal transmissions involved.
Now, if such a video communication system using fully digital satellite transmissions is to be realized by utilizing the FDMA (Frequency Division Multiple Access) mode, it would become necessary to expand the available frequency band considerably in order to avoid the mutual interference among different channels, whereas if such a video communication system using fully digital satellite transmissions is to be realized by utilizing the TDMA (Time Division Multiple Access) mode, it would become necessary for each station to be provided with an expensive high power amplifier equipment such as TWTA (Travelling Wave Tube Amplifier), and this can lead to a very large-sized earth station.